This invention relates to friction welders, and more particularly to servo hydraulically driven friction welders.
Although the invention was developed in the field of aircraft engines it has application to other fields where friction welding may be used to accurately and effectively bond two elements together.
Friction welding is a well-known process in which two components, moving relative to each other, are brought into contact under pressure and bonded at their interface. The motion at the weld interface may be rotational or non-rotational. Non-rotational motion includes linear, elliptical or vibratory motion. Friction welding by rotational motion typically requires at least one of the components be circular in cross section. However, friction welding by non-rotational motion has received attention as a means of bonding components, where the weld interface of both parts is non-circular.
In non-rotational friction welding, one component is oscillated relative to the other component while a forge force is applied normal to the direction of motion. The forge force moves the components into contact, and with metal components the friction between the components generates heat and plasticizes them. Once the motion stops, the metal solidifies, thus bonding the components. This relative simplicity of the process, as compared to other welding processes, lends itself to methodologies that permit tight control of the weld process. Rigid process control may eliminate the necessity of post-weld inspection of the components. Weld parameters such as frequency and amplitude of oscillation, axial displacement, and normal force can be precisely monitored and controlled to produce consistent and repeatable welds.
For plastic components, the friction weld process is typically performed at high frequencies and low forge forces. An example of a process for friction welding thermoplastic components is disclosed in U.S. Pat. No. 4,377,428, issued to Toth and entitled xe2x80x9cMethod of Friction Weldingxe2x80x9d.
However, for metal components, the conditions required for friction welding are much more stringent. In addition, there are large forces associated with friction welding metal components. Typically, for metal components the oscillation frequencies are less than/about 100 Hz, depending on the part size and shape, and the forge forces are greater than 5000 lbs. force. A welder having substantial structure is needed to withstand the larger forces associated with friction welding of metal components. Due to the size of such structures, interference between the oscillation frequency and the resonant frequency of the welder is a concern. In addition, repeatability of the process is necessary for process control. Repeatability requires the final position of the components, when welded, to be accurate and predictable.
The actuation system used to generate the oscillating motion must be able to provide a consistent frequency and amplitude and be able to locate the oscillated component in the proper position for forging. One type of actuation system is a mechanically driven system such as that disclosed in U.S. Pat. No. 5,148,957, issued to Searle and entitled xe2x80x9cFriction Weldingxe2x80x9d. In this type of actuation system, cams and joints are used to provide the reciprocating motion. A drawback to mechanically driven actuation systems is the wear, which occurs in the system components. As the system is used, the cams, joints, and bearings will wear which will result in deviations that have to be accounted for, to ensure accuracy and repeatability of the process. Eventually the wear will require replacement of worn parts, which introduces an additional deviation to be accounted for. The actuation system will require re-calibration frequently to account for all the deviations. Another example of a mechanically driven actuation system is disclosed in U.S. Pat. No. 4,858,815, issued to Roberts et al and entitled xe2x80x9cFriction Welder Mechanismxe2x80x9d.
Another type of actuation system is a servo-hydraulically controlled actuation system, such as that disclosed in U.S. Pat. No. 4,844,320, issued to Stokes et al and entitled xe2x80x9cControl System and Method for Vibration Weldingxe2x80x9d. One limitation to known types of servo-hydraulics is the interference between the oscillating frequency and the natural frequency of the hydraulic column. To generate low frequencies ( less than 100 Hz) and the forge forces needed to move metal components subject to a normal force, the hydraulic columns needed are typically large enough to have natural frequencies of the same order of magnitude as the oscillating frequency.
A particularly useful application for which friction welding is useful is in fabricating integrally bladed rotors for gas turbine engines. An example of this type of application is disclosed in U.S. Pat. No. 5,035,411, issued to Daines et al and entitled xe2x80x9cFriction Bonding Apparatusxe2x80x9d. An integrally bladed rotor is a rotor assembly wherein the rotor blades are bonded, typically by welding, directly to the rotor disk at spaced intervals about the circumference of the disk. Since there are numerous rotor blades bonded to each disk, the bonding process must be accurately repeatable. In this way individually manufactured components each with selected properties may be joined. Each bonded blade must be accurately positioned within tight tolerances required for aerospace applications. An improved friction welder and method are sought for friction welding large scale, complex shapes formed from various metallic materials.
The present invention is predicated in part upon the recognition that non-planar forces, relative to the plane of the forge pressure, cause deviations in the location of the parts being bonded. The deviation degrades the accurate repeatability of the welding process.
According to the present invention, a friction welder for joining a pair of elements includes a forge assembly having a table defining a platform for disposing one of the elements thereon and wherein, during the application of forge pressure between the elements, non-planar forces in the platform are minimized. Forge pressure is generated between the elements by a forge link disposed on one side of the platform and reacted by a reaction link disposed on the opposite side of the platform. The two links are equidistant from the plane of the platform and connected by a crank that is also pivotally connected to the table.
A principle feature of the present invention is the table having a reaction link disposed opposite of the forge link. The advantage produced thereby is the accuracy of the welded position between the two elements as a result of the minimized non-planar forces in the platform. Minimizing or eliminating non-planar forces in the platform results in minimizing or eliminating deflections and deviations in the relative positions of the elements being welded. Another advantage is the accurate repeatability of the process as a result of the control over deflections and deviations in relative position.
According to another embodiment of the present invention, a method of friction welding a pair of elements includes the step of balancing the moments in the table such that non-planar forces in the platform are minimized. In a specific embodiment, the method includes the steps of placing the forge link in a bent position such that the elements may be positioned in the welder and placing the forge link in the forge position such that forge force may be applied to the elements. The step of balancing the moments and deflections in the table prior to reciprocating the elements improves the accuracy and repeatability of the weld process.
According to a further embodiment, the forge link includes a first end, a second end, both of which are disposed on a forge axis, and a pivoting joint therebetween. The pivoting joint permits the forge link to be flexed such that the table may be moved away from the point of engagement between the two elements.
The feature of the forge link having the pivoting joint results in the advantage of ease of assembly of the elements into the friction welder. The pivoting joint permits the table to move away from the point of engagement to thereby provide access to the tooling and gripper.
According to a specific embodiment of the present invention, the friction welder includes a frame having a base, a pair of vertically extending trusses interconnected at one end and to the base at the opposite end, and a diagonal truss extending between the vertical trusses and the base. A reciprocal motion assembly is disposed on the diagonal truss. The forge link extends between the diagonal truss and the crank.
A further feature of the present invention is the capability to use a servo hydraulic control system used in the reciprocating motion assembly. The advantage is the improved control of the reciprocal motion assembly for reciprocating frequencies of less than 100 Hz. The stiffness of the frame and forge assembly and the minimal length of the hydraulic column within the servo hydraulic control of the reciprocating motion assembly results in a natural frequency for the structure servo hydraulic control in excess of the reciprocating frequencies required.